We have had numerous complaints about particular Big Stoppers being faulty, causing light leaks and strange magenta colour casts on the image.
In every case so far, this has been caused by user error. The viewfinder MUST be covered during long exposures on both film and digital cameras. If left uncovered, light will ingress and find its way to the sensor, causing the flare lines and magenta casting that seem to indicate a faulty filter.
The filter must be placed in the rearmost filter slot, with the foam making a seal onto the filter holder body.
It must also be used in a LEE filter holder - other makes of holder do not have an adequate design to ensure a light free seal.
The viewfinder must be covered during all long exposures, regardless of the amount of ambient light.
We hope that this helps.